(1) Field of the Invention
This invention relates generally to electronic displays and relates more particularly to gamma curve correction for display modules.
(2) Description of the Prior Art
The gamma curve is a mathematical function that describes the non-linear tonal response of many monitors. A tone map that has the shape of this its compensating function cancels the non-linearities in monitors.
The luminance of a liquid crystal display (LCD) is dependent on the voltage across the liquid crystal and the properties of the liquid crystal itself. The use of liquid crystals from different suppliers as well as production lot variations affect the luminance of a LCD display. The LCD display module manufacturers supply the settings for the gamma curve of the display drivers of their displays to adjust for typical LCD display parameters. These typical gamma curve parameters are then loaded into the application program of the related LCD display drivers.
Variations of display module parameters cause variations of luminance of individual displays. It is sometimes necessary to adjust the application program of an LCD display driver to reflect the specific LCD display parameters.
There are various patents known to adjust the gamma curve of LCD displays:
U.S. Pat. No. 6,359,389 (to Medina et al.) describes a flat panel display having a programmable gamma without incidental loss in grayscale resolution. In one embodiment, the flat panel display is a liquid crystal display (LCD). The invention includes applying and adjusting a set of gamma controlling voltages to the DC reference circuit (a.k.a. ladder voltages) of an LCD module producing a change in the gamma response (or profile) of the LCD module without incidental loss of grayscale resolution. An adjustable ladder circuit (ALC) is thereby realized. Separate ALCs can be provided for red, green and blue primaries. By adjusting, in a predetermined fashion, the reference voltages applied to the row and column drivers of an LCD display, the gamma response of the LCD can be changed to a different value. Because the input digital signals are not affected, the same color resolution and dynamic range are maintained. The DC reference circuit can be a multi-node voltage divider. These voltage nodes are applied to the row and column drivers of the LCD module to control the ON/OFF states of each red, green and blue sub-pixel. The input digital signals provided by the host's graphic source or software application modulate these voltage nodes to produce the desired grayscale value applying across the LCD sub-pixel a percentage of DC reference voltages.
U.S. Pat. No. 6,437,716 (to Nakao) discloses a grayscale display reference voltage generating circuit that can change a gamma correction characteristic in accordance with a liquid crystal material and LCD panel characteristics. Resistor elements R0 through R7 have a resistance ratio for gamma correction and generate gamma-corrected intermediate voltages on the basis of voltages across both input terminals V0 and V64. A gamma correction adjustment circuit 42 adjusts the gamma-corrected intermediate voltages upward or downward on the basis of adjustment data latched in a data latch circuit 43. By thus supplying the adjustment data corresponding to the liquid crystal material and the LCD panel characteristics to the data latch circuit 43, the gamma correction characteristic can be changed in accordance with the liquid crystal material and the LCD panel characteristics without modifying the design of a source driver.
U.S. Pat. No. 6,731,259 (to Yer et al.) discloses a driving circuit of an LCD device compensating a gamma voltage according to a peripheral environment so that exact picture images can be displayed. The driving circuit of the LCD device includes a memory dividing the peripheral environment into a plurality of modes and storing information of each mode, an environment sensor sensing variation of the peripheral environment, a controller selecting information of a mode corresponding to the resultant value sensed by the environment sensor, a digital variable resistor adjusting a resistance value to correspond to mode information selected by the controller, and a gamma voltage outputting unit outputting a plurality of gamma voltages corresponding to the adjusted resistance value.